An electrophotographic image forming apparatus forms a color image by superimposing toner images, in respective colors carried on photoreceptors, on each other on the intermediate transfer belt in a primary transfer section and then secondarily transfers the superimposed toner image from the intermediate transfer belt to a transfer material. In such an electrophotographic image forming apparatus, the amount of electrical charge of toner on the intermediate transfer belt varies with the number of times, environment or the like of performing the primary transfer. Accordingly, various image defects tend to occur at the time of the secondary transfer from the intermediate transfer belt to the transfer material.
In this situation, there are offered technologies that uniforms the amount of electrical charge of a toner image by charging of a toner image having been primarily transferred to an intermediate transfer belt, with corona discharge from a pre-secondary transfer charging device (for example, refer to Japanese Unexamined Patent Application Publication Nos. H10-274892 and H11-143255).
On the other hand, in order to prevent irregularity of density due to an electrical charge shortage for transfer which occurs when the toner adherence amount is large and the electrical potential of the toner layer is high, and also to prevent a discharge which is caused by increasing the transfer charge amount, the inventors are considering a technology to electrically neutralize a toner image on an intermediate transfer belt prior to secondary transfer.
Specifically, a toner image on an intermediate transfer belt is neutralized by installing a scorotron electrode on the upstream side of the secondary transfer. Herein, it is preferable to employ an elastic member of which surface is a conductive brush, conductive sponge, or the like, for the counter electrode which is pressed to contact with the intermediate transfer belt so that tight-contactability with the back surface of the intermediate transfer belt is improved.
However, when an elastic member is pressed on a belt, instability of the surface of the running belt affects the downstream side of a nip section for the primary transfer and the upstream side of a nip section for the secondary transfer, causing a problem of instability of image quality due to splashing of toner on a periphery of a character or thin line or roughness of an image caused by irregular discharge.
An object of the invention is to provide an image forming apparatus that prevents problems, as described above, and maintains a stable image with a regulating member for regulating the position of an intermediate transfer belt surface.